This application claims the priority of German Patent Document 100 25 496.9, filed May 23, 2000, the disclosure of which is expressly incorporated by reference herein.
The invention relates to an audio system, in particular for motor vehicles. Such an audio system is known, for example, from U.S. Pat. No. 5,434,922 which includes a loudspeaker installation for supplying sound to an audio chamber, e.g. a vehicle interior, and also a control unit which produces control signals for the purpose of operating the loudspeaker installation. In this case, this control unit is equipped with a correction device and has a signal detection unit which receives microphone signals from a microphone exposed to the audio chamber and forwards them to an evaluation unit which extracts interference signals from the microphone signals. The control unit then produces the control signals for operating the loudspeaker installation on the basis of these interference signals.
Such an audio system is used for intentionally supplying sound to an audio chamber in order to convey a particular acoustic impression to a listener situated in the chamber. To this end, the audio system contains at least one audio signal source, e.g. a radio, a CD player, a telephone installation, which provides audio signals on the basis of which the control signals for operating the loudspeaker installation are produced. The acoustic impression of the intentionally produced sound waves can be disrupted by noise, however. In a vehicle, the noise which disrupts the audio impression in the vehicle interior may be formed, by way of example, by noise from the tires on the ground, wind noise or noise created by an open window or an open sunroof. To be able to achieve the highest possible quality for the listener""s acoustic impression, modern audio systems are equipped with a correction device which uses microphones to detect the sound in the audio chamber and extracts the noise or the interference signals correlating thereto from that sound. On the basis of the interference signals ascertained, the control in the audio system can then introduce suitable measures which reduce the influence of the noise on the acoustic impression for the listener and which improve the quality of the desired acoustic impression.
To be able to implement such an audio system in an audio chamber, e.g. in a vehicle interior, at least one microphone needs to be installed in the audio chamber in order to be able to detect the actual acoustic condition in the audio chamber permanently. The greater the number of arranged microphones, the more precise the actual acoustic condition in the audio chamber can be ascertained. However, installation of these additional microphones means an increased level of complexity when fitting such an audio installation.
EP 0 539 939 B1 discloses the practice of setting up the microphones in the vehicle interior in conjunction with one of the loudspeakers in the loudspeaker installation. In this way, microphone and loudspeaker form a standard assembly which can be fitted in the vehicle in unison. This allows the fitting complexity to be reduced.
The present invention is concerned with the problem of specifying, for an audio system of the type mentioned in the introduction, another way in which it is possible to reduce the fitting complexity for installation of the audio system.
The invention is based on the general concept of using or operating at least one tweeter in the loudspeaker installation as a microphone. In this context, the invention is based on the realization that, on the one hand, the noise which has a particularly intensive disruptive effect on the acoustic audio impression in a vehicle interior, in particular, is situated in a relatively low frequency range, whereas, on the other hand, the sound waves primarily radiated by a tweeter are situated in a relatively high frequency range. This means that a crossover frequency after which the tweeter is involved in the radiation of sound in the loudspeaker installation is situated above the frequency spectrum of the critical noise. On the basis of this realization, the invention proposes connecting the respective tweeter such that it operates as a loudspeaker above its crossover frequency and operates as a microphone below its crossover frequency. While relatively high-frequency electrical voltages entering in loudspeaker mode are converted into corresponding, relatively high-frequency sound waves and are radiated, relatively low-frequency sound waves arriving at the tweeter or at the diaphragm thereof prompt conversion, as a result of inductive processes, into corresponding, relatively low-frequency electrical voltages which can be picked off at the connections of the loudspeaker. These voltages form electrical interference signals correlating to the acoustic noise.
The advantages of such an embodiment are obvious, since no separate microphones need be provided, in which case the additional fitting of microphones is also dispensed with. The circuit or electronics required for producing the audio system according to the invention is less expensive than the microphones which it replaces and may be implemented during the actual manufacture of the audio system.
In accordance with one advantageous embodiment, the tweeter can be connected via a high-pass filter circuit to an output side of a loudspeaker amplifier whose input side receives the control signals from the control unit. As a result of this measure, no shorting is produced at the output of the loudspeaker amplifier during induction of the low-frequency microphone signals. By way of example, this high-pass filter circuit can be produced by passively decoupling the tweeter from the loudspeaker amplifier using a capacitor.
In accordance with one preferred development, the tweeter can be connected via a low-pass filter circuit to an input side of a microphone amplifier whose output side forwards the microphone signals to a signal detection unit. This embodiment ensures that the high-frequency control signals for the loudspeaker amplifier are not applied to the microphone amplifier.
To simplify the processing of the microphone signals and, in particular, to allow the processing to be carried out using a microprocessor, the signal detection unit preferably has an analog/digital converter which converts the incoming analog microphone signals into digital microphone signals and forwards them to a digital evaluation unit.
In accordance with one preferred embodiment, the control unit is able to control and/or regulate at least one of the following functions on the basis of the interference signals: equalizing, compression, limiter, level matching, filter matching. This allows, by way of example, dynamic equalizing to be achieved, where equalizing quality, equalizing gain and equalizing frequencies are influenced on the basis of the interference signals. Similarly, dynamic compression and a dynamic limiter function can be obtained, where, by way of example, a compression factor, a limiter threshold and the control times for attack and decay are accordingly set on the basis of the interference signals. In addition, dynamic level matching, i.e. interference-signal-dependent volume adjustment, and dynamic filter matching can be achieved, where, by way of example, the crossover frequencies, the gain factors and shape factors for bass filters, midrange filters and treble filters are set and readjusted on the basis of the interference signals.